1. Field of the Invention
This invention relates to apparatus for mounting a detector head including a collimator onto the supporting structure of a radiation detecting device used for medical diagnostic purposes.
2. Description of the Prior Art
Radiation detectors, such as nuclear radiation detectors like scintillation cameras, are widely used to develop information for medical diagnostic purposes based on signals derived from a source of radiation. Such radiation detectors are used in noninvasive medical diagnostic procedures wherein a head of the detector pivotally mounted onto supporting structure is positioned to face a part of the patient under study (e.g. a body organ) at all times. A commonly used radiation detector for such purposes is an Anger-type scintillation camera (named for its inventor), the basic principles of operation of which are disclosed in U.S. Pat. Nos. 3,011,057; 3,732,419 and 3,984,689. The radiation detector computes the distribution of the radiation emitting substance previously ingested by the patient as detected by the detector head from its viewing position, and analyzes this data to produce diagnostic information about the object of study. This is done by determining the distribution of the radiation emitting substance in the human body organ by analyzing the locations of scintillation events which occur on a scintillation crystal due to rays incident from the body organ. The Anger camera and other radiation detectors typically employ a radiation collimator between the radiation sensitive transducer (e.g. the crystal in the Anger camera) and the source of radiation.
The purpose of using a radiation collimator is to provide radiation transmissive passageways to ensure a mapping correspondence between respective elemental volumes of the radiation source (e.g. the body organ) and elemental volumes of the transducer (e.g. the Anger camera crystal). The most commonly used collimator is of a multi-channel type which comprises a number of radiation transmitting apertures or channels separated from each other by radiation opaque walls or septa. The collimator ensures that only rays traveling parallel to the radiation transmitting channels of the collimator will be transmitted from the patient to the radiation transducer; the passage of other rays will be blocked. The choice of collimator to be used with the detector head in a particular medical diagnostic procedure, depends on the energy level of the radiation emitting substance being used.
It is well known that radiation collimator design involves basically the parameters of aperture size and shape, septal thickness, and aperture length. These are the parameters which determine the resolution and the efficiency of the collimator for radiation (e.g. gamma rays) of a particular energy. In general, the septal thickness, which is the thickness of the walls separating adjacent collimating apertures, is chosen in accordance with the energies of the rays to be collimated so that the collimator will block the rays which enter the collimator at an angle and location such that they would otherwise traverse the wall between two apertures. Thus, the septal thickness must be relatively large for collimators used with high energy radiation sources, but for low or medium energy sources the septum or wall between the apertures may be quite thin. It is desirable to employ only the septal thickness actually required for the radiation energy involved in order to avoid unecessary loss of efficiency.
The supporting structure for a radiation detector, such as an Anger-type scintillation camera, generally includes a base on which is located a height-adjustable support arm having a yoke between the bifurcated ends of which is received the detector head. The head is positioned in the yoke so that it may be oriented into a desired position relative to the patient. The detector head, including a collimator selected for the intended application of the detector, is pivotally mounted onto the supporting structure for rotation about a single trunnion axis running through the respective pivot points of attachment of the head to the respective ends of the yoke. Brake or other locking means is provided to fix the position of the head relative to the supporting structure yoke after the desired positioning has been achieved. For ease of adjustment and greater patient safety, it has been found advantageous to mount the detector head onto the supporting structure so that the trunnion axis about which the head rotates coincides with the center of gravity of the head including the collimator. Such balancing is especially desirable where the supporting structure is adapted to rotate the head to provide emission computerized tomography (ECT), in which the detector head precesses about the patient to produce a display showing the radioactive distribution in the object of study in a number of parallel section imaging planes. Prior art structure mounting the detector head for emission computerized tomography is disclosed in U.S. Pat. No. 4,216,381. Commonly assigned, copending application Ser. No. 273,446 filed June 15, 1981, by Haas, et al., entitled "Improved Apparatus for Driving a Radiation Detector" shows newly proposed supporting structure mounting the detector head for ECT application.
A disadvantage of existing mechanisms for mounting a detector head relative to supporting structure of a radiation detector, wherein the detector head is mounted for pivotal movement about a single trunnion axis, is the inability to maintain the detector head in a balanced configuration for different weight collimators. Low energy collimators, for example, may be approximately 80 lbs. lighter than medium energy collimators. Thus, where a detector head including a collimator is pivotally mounted onto the supporting structure of a radiation detector so that the center of gravity of the head is coincident with the single trunnion axis, changing collimators can result in an unbalanced detector head because of the weight differential for different collimator types. An unbalanced head is undesirable for operational ease and patient safety reasons.